1. Field of the Invention
This invention relates to the purification of air by scrubbing it in contact means such as a scrubber utilizing an effective scrubbing liquid which can be reconditioned for reuse.
2. Problem
Air pollutants are of particulate character, of gaseous character, or both, and such pollutants result principally from industry or combustion. Industrial sources include ventilation products from local exhaust systems and process waste discharges, as well as from combustion. Air pollutants may be more or less odorous and/or pungent.
Known pollution control systems do not completely or effectively remove certain pollutants such as those arising from paint booths, plastics fabricators and some petroleum processes.
Emissions from sewage and sludge treatment, restaurants, renderers, brewers and distillers, fish reduction and processing plants, pulp and paper mills, fertilizer processors and manufacturers, poultry and poultry processing operations, coffee roasting, garbage and garbage incineration, paint booths, enamel baking, internal combustion exhaust, plastics industries, petroleum processes, smelting and mining operations, swine, and other domestic food processing operations, various chemical operations and operations such as printing where odoriferous materials are employed are amenable to treatment at least to some extent by the present invention.
A particular problem is to remove styrene and related products from air.
Technology which may be effective for deodorizing in some types of operation is prohibitively expensive for use in small plants. Thus, a catalytic afterburner that can be used for controlling methyl styrene has both a high basic cost and large operating expense.
Air pollutants must be separated from the scrubbing liquid if the scrubbing liquid is to be reused, which is economically desirable, or if the spent scrubbing liquid is to be discarded in a modern sanitary sewer system and such materials are detrimental to sewage treatment or disposal.
Prior Art
The objective of prior art air-cleaning processes has been to remove particulate matter and/or noxious gases and pungency. To remove particulate material, cyclone separators, bag filters and electrostatic precipitators have been used. In wet scrubbers, water is sprayed in atomized condition over a geometric shape to remove water-soluble gases. Chemicals may be added to the liquid to provide improved absorption. These devices provide for mixing a gas stream under treatment with water or a chemical solution so that gases are taken into solution and possibly converted chemically as well. Filters packed with solid absorbents or adsorbents are used to sorb impurities from air.
A typical cyclonic spray scrubber is shown in FIG. 2 and a typical venturi scrubber is shown in FIG. 3 on page 241 of Volume 1 of the McGraw-Hill Encyclopedia of Science and Technology, 1971 edition. A cyclone dust separator, a cloth collector and an additional view of a cyclonic liquid scrubber are shown on pages 333 and 334 of Volume 4 of the same publication.
Packed beds providing a bed or layer of packing composed of granular material such as sand, coke, gravel and ceramic rings, or fibrous materials such as glass wool and steel wool are used to remove particulates. Packed towers, plate towers and spray towers, as well as filters are used to adsorb gases.
The same encyclopedia points out at Volume 4, page 334, column 1, that packed beds, because of a gradual plugging caused by particle accumulation, are usually limited in use to collecting particles present in the gas at low concentration unless some provision is made for removing the dust, for example by periodic or continuous withdrawal of part of the packing for cleaning.
The same encyclopedia states in column 2 of the same page that an extremely wide variety of scrubbing equipment is available, ranging from simple modifications of corresponding dry units to permit liquid addition, to devices specifically designed for wet operation only.
In column 2 on page 47 of Volume 6 of the same encyclopedia it is stated that recovery of solute gases is generally achieved by contacting the gas streams with a liquid that offers specific or selective solubility for the solute gas to be recovered. By such procedure mercaptans can be removed from natural gas, or carbon monoxide can be recovered from process streams in petrochemical synthesis. Fluorides can be recovered from emissions from fertilizer and aluminum manufacture and sulfur dioxide can be recovered from metallurgical operations. FIG. 1 on page 48 shows a diagram of a packed tower for gas absorption, FIG. 2 shows a wet cyclone peripheral spray tower and FIG. 3 on page 49 shows a plate tower or impingement baffle scrubber. Page 49 states that the packed tower is used most extensively in absorption operations because of its inherent differential mechanism and low power consumption.
Pages 205 and 206 of Volume 13 of the same encyclopedia states that volatile material can be stripped from scrubbing liquid in order to permit recovery of the absorbed solute and recycling of the solvent. Such stripping can be accomplished by pressure reduction, the application of heat or the use of an inert gas as a stripping vapor or some combination of such procedures. Steam can be used as a stripping vapor for a system which is not miscible with water.
Gladu U.S. Pat. No. 3,660,045, issued May 2, 1972, shows an air-purifying scrubber for removing both gaseous pollutants and particulate pollutants, especially from smoke. Nozzles project jets of combustible gas into the polluted air to heat fumes, such as from a pulp mill, paper mill or smelter, so as to remove or convert the odorous components of the smoke into unobjectionable fumes as stated at column 4, lines 36 to 45. Subsequently, the fumes and smoke are passed through a washing zone into which nozzles project water sprays (column 4, lines 62 to 65). The heating and combustion will complete the oxidation of carbon compounds of the gas and the oxidation or conversion of other noxious and/or odorous components of the gas, such as those containing sulfur (column 4, lines 73 to column 5, line 3). The water from the water sprays will both entrain particulate material and dissolve to a greater or lesser extent water-soluble components of the smoke (column 5, lines 6 to 9).
As stated in column 2, on page 242 of Volume 1 of the McGraw-Hill Encyclopedia referred to above, in incineration of municipal garbage volatiles are driven off by destructive distillation and ignite from the heat of the combustion chamber. The gas is passed through a series of oxidation changes but must be heated above 1200 degrees F. to destroy odors. The end products of the incineration pass through expansion chambers, fly ash collectors, wet scrubbers and, in some instances, electrostatic precipitators. The end products include carbon dioxide, carbon monoxide, water, oxides of nitrogen, aldehydes, unoxidized or unburned hydrocarbons and particulate matter comprising unburned carbon, mineral oxides and unburned refuse.
Drawert U.S. Pat. 4,426,210, issued Jan. 17, 1984, was concerned with reducing odor emissions from industrial waste gases from factories of numerous types concerned with industrial food processing, for example, breweries, malting houses, coffee roasting plants, fish processing plants, soup seasoning installations and from factories concerned with the disposal of animal carcasses (column 1, lines 1 to 15). Drawert was especially concerned about odor-emitting substances encountered in breweries during preparation of the mash and boiling of the worts (column 1, lines 21 to 23). This patent recognized that effluent gas had been washed by ozone or chloride of lime (column 1, lines 39 to 41) or had been processed by a variety of adsorption and absorption filters such as biological filters and active carbon (column 1, lines 44 to 46). Drawert proposed to use wash water sprays containing nonionic compounds including partial esters of polyalcohols or ampholytic compounds such as long-chain substituted amino acids or betaines or mixtures of these compounds (column 1, line 64 to column 2, line 11). Drawert contended that the additions of these surface-active substances to the washing water provided solubility for the odor-emitting substances to be removed (column 2, lines 17 to 29). This procedure however is quite different from the procedure of the present invention.
European patent application No. 0 073 171 proposed to remove a wide variety of gaseous contaminants from a gas stream such as generated by industrial manufacturing, storage or commercial processes or operations (page 11, lines 20 to 24) by providing a first absorption system for water-soluble contaminants such as methanol, ethanol, isopropanol, acetone, methyl acetate, mercaptans, glycols, acetic acid, isopropyl ether, pyridine, acetonitrile and dioxane (page 8, line 20 to 24) by using absorption liquids including ethylene glycol, propylene carbonate, butoxy diethylene glycol acetate, acetic acid, high boiling point alcohols, Carbowaxes, polyethylene glycols and polypropylene glycols (page 8, line 32 to page 9, line 5). The gases were then treated by a second absorption liquid in the form of a water-immiscible oil or mixture of oils which is a nonsaponifiable, nondrying mineral, synthetic, paraffinic, animal or vegetable oil of high boiling point as stated at page 9, lines 7 to 20. This second absorption liquid is used to remove gaseous contaminants which are not soluble in the first absorption liquid (page 9, lines 30 to 32). This publication does not suggest the use of unsaturated or drying oils.
Yokogawa U.S. Pat. No. 4,528,001, issued July 9, 1985, is particularly concerned with volatile organic matter consumed or handled as fuels or solvents in various fields of industry, such as the petroleum depot (gas station), petroleum refining plants, painting factories, printing factories, rubber processing factories and general chemical plants where large amounts of volatile organic solvents and vehicles are consumed (column 1, lines 37 to 41). This patent is particularly concerned with volatile organic matter that is insoluble or only slightly soluble in water (column 3, lines 35 to 40). The organic matter to be recoverd from a gas or gas mixture include crude oil, refined petroleum fractions such as petroleum ether, benzene, ligroin, gasoline, light oils, aliphatic hydrocarbons, aromatic hydrocarbons, halogenated hydrocarbons, esters, ketones and ethers listed specifically at column 4, lines 40 to 54. Such pollutants are proposed to be captured by a homogeneous aqueous solution capable of dissolving such pollutants including hemiethers and hemiesters of polyalkylenepolyols specified at column 4, line 68 to column 5, line 20.
While, as stated above, U.S. Pat. No. 4,528,001 mentions painting factories among others, Mir U.S. Pat. No. 4,353,715, issued Oct. 12, 1982, Cosper U.S. Patent No. 4,378,235, issued Mar. 29, 1983 and Japanese Patent Publication No. 51-94489 are all concerned with removing organic solvents from paint spray booths. As stated in U.S. Pat. No. 4,353,715, paint spray booth and paint dryer exhaust gases may contain volatile hydrocarbons, esters, ethers and alcohols such as acetone, methylethyl ketone, toluene, xylene and ethyl acetate (column 1, line 17 to 21). The Cosper patent at column 4, line 67 to column 5, line 2, specifies similar pollutants and in addition mentions methylamyl ketone, petroleum naphthas and diacetone alcohol.
U.S. Pat. No. 4,353,715 uses an oil-in-water emulsion containing a surfactant (column 3, lines 7 and 23). The oil-in-water emulsion scrubbing media comprises a hydrocarbon oil such as a long-chain hydrocarbon or lubricating type of oil in which the solvent from the paint-spray booth is soluble (column 4, lines 18 to 22). The surfactant may be a nonionic, anionic or cationic surfactant containing an oleophilic portion as stated at column 3, lines 19 to 22. The surfactant may be of the petroleum-sulfonate type selected to have adequate solubility for the vapor components to be absorbed as stated at column 5, lines 45 to 48.
U.S. Pat. No. 4,378,235 also advocates the use of an oil-in-water emulsion to sequester paint spray solvents, utilizing a hydrocarbon oil as stated at column 2, lines 17 and 66 and column 5, line 2, which hydrocarbon oil must act as a solvent for the particular volatile organic paint carriers as stated at column 5, lines 2 to 4. Various examples of hydrocarbon oils are given in the seventy-four examples of Table I in columns 5, 6, 7 and 8. The present invention does not use hydrocarbon oil.
To emulsify such hydrocarbon oils of U.S. Pat. No. 4,378,235 saturated and unsaturated fatty acids and alkali salts thereof are used as emulsifiers as stated at column 3, lines 52 to 54, various examples being given in Table I, including oleic acid, sulfonic acid, chlor. stearic acid, stearic acid, palmitic acid, myristic acid, lauric acid, tall oil acid, tallow acids, dioleate ester and fatty alcohol sulfate. Such surfactants are used in proportions from 1.8% to 20%, except that in one instance, Example 37, oleic acid alone was used without any hydrocarbon oil. The percent of reduction by the use of oleic acid alone was 48.2% in ten minutes and 42% in twenty minutes which was a considerably lower reduction than that achieved by many of the other examples given using hydrocarbon oil.
The Japanese Patent Publication No. 51-94489 contacts the organic solvent gas from a paint booth with odorless high-boiling point oil in order to dissolve the organic solvent gas into the oil. The gas is squirted through a body of the oil which may be vegetable oil such as olive oil and castor oil but mineral oils can also be used. Olive oil and undehydrated castor oil are not drying oils or semidrying oils or polyunsaturated oils such as used in the process of the present application. The organic solvents can be separated from the oil by centrifuging.
Japanese Patent Publication No. 57-153725, 1982, is particularly concerned with deodorizing malodorous gas generated in the treatment of human and animal feces and other waste such as garbage. The treatment involved passing the gas through a body of heated oil such as animal oil, vegetable oil or mineral oil in a deodorization tank under low pressure. After passing through the tank, the gas is again contacted with a spray of the oil removed from the tank and returned to the tank. This process is purported to remove from the air compounds such as indol and scatol. The process does not propose to use unsaturated or drying oils as used in the invention of this application.
Thus various procedures have been used to purify air with greater or less success but conventional procedures have been unable to remove certain air pollutants, particularly styrene chemicals. British Pat. No. 548,908 of 1942 makes reference to removing gases containing hydrocarbons resulting from processing coal gas and specifically mentions styrene at page 1, lines 21 and 56. Previously it has been the common practice to scrub the gas with large quantities of mineral oil to remove undesirable constituents and recover light oils (page 1, lines 18 to 24) and the invention of that British patent involved incorporating in the mineral scrubbing oil of a small quantity of one or more alkyl aminophelols (page 1, lines 69 to 73). The scrubbing oil was brought into contact with the gas by countercurrent flow in a column or by spraying the gas with oil (page 1, lines 82 to 86). This patent also proposed the use of a relatively stable and inert organic liquid having a boiling point not substantially below 250 degrees centigrade as a substitute for mineral oil at page 2, lines 42 to 47. It is applicant's experience that mineral oil is not effective to remove styrene pollutant from air.
Air-filtering processes generally have required the expenditure of considerable power, or have required the utilization of expensive equipment and/or materials, or have been of limited capacity, or have required frequent and expensive reconditioning of sorption materials.
Adsorption beds packed with activated carbon, activated alumina, activated bauxite, acid-treated clays, iron oxide, magnesia and fuller's earth have been used to adsorb gases as stated in the Kirk-Othmer Encyclopedia, Second Edition, Volume 1, page 460. Such solid materials are generally expensive so that it is the practice to revitalize them to provide regenerated adsorptive material and this operation must be performed frequently because of the limited adsorptive capacity of such substances. The bed material must be physically removed and transported from the location of the filter bed to the reprocessing equipment. The regenerating operation results in significant physical loss of the filter bed material, as well as the material adsorbed by the filter material being lost.
In some installations, instead of using solid filter material, chemically active liquids are used in a scrubber. In order for such scrubbing solutions to be effective, intricate and large applicator equipment is employed to provide sufficient residence or contact time for the purging chemical reaction to occur.
To reclaim liquid scrubbing media, copious water washing, filtration, gravity settling, steam stripping, sonic or ultrasonic treatment, reaction with one or more of selected reactants and/or exposure to ultraviolet radiation may be utilized to effect precipitation and removal of the sorbed pollutants from the scrubbing liquid to prepare it for reuse.